Export listings as Facebook Marketplace bulk-upload XLSX

Adds a "download for facebook" link on each admin edit page and a multi-select mode on the admin listings page so the admin can pick up to 50 listings and download them as a single XLSX matching Facebook Marketplace's bulk-upload template. The export route loads Facebook's own template (preserving the hidden VALIDATION sheet that drives the CONDITION / OFFER-SHIPPING dropdowns), fills rows in selection order, defaults missing condition to "Used - Good" (FB requires a value), and returns typed 400/404 errors rather than leaking the template's sample item on stale or empty selections. Also moves the edit-page save button into the top-right header so it stays visible while scrolling through images, pins vite dev to port 5182 to avoid port collisions, fixes the broken pnpm-workspace.yaml placeholder so better-sqlite3 / sharp / esbuild actually build on install, and ignores local.db* plus .playwright-mcp/.